TT virus has been shown to be transmitted to frequent blood transfusion recipients, posing a potential threat to blood safety. In studies conducted in this laboratory, TT virus DNA was found in 2 of 47 serum samples from Japan. To investigate the prevalence of TT virus in the United States, we screened 100 serum samples collected from leukaphresis donors at NIH. DNA was extracted and purified DNA was used as a template to amplify a TT virus sequence by two rounds of PCR. 22 of the 100 samples were tested positive by PCR (22%). The specificity and sequence divergence of the PCR positive samples were confirmed in 9 of the 22 samples by DNA sequencing and by comparing the results with the published TT virus information. These results confirm the presence of the TT virus in the US population with diversified DNA sequences.